Up to 98,000 patients die each year in the United States due to medical error. While nurse and physician-in-training sleep deprivation has been found to significantly increase the risk of medical errors and occupational injuries, very little information has emerged on the relationship between senior (attending) physician work hours and patient safety. In surgery and obstetrics, the hours of attending physicians can match or exceed those of physicians-in-training. Whether such long work hours might degrade the performance of experienced attending physicians as severely as that of resident-physicians, however, is unknown. Experienced attending physicians may be less prone to attentional failures and performance decrements than younger physicians-in-training who have a stronger physiologic drive to sleep when awake for long hours. Conversely, sleep quality tends to deteriorate with age, which could lead to a buildup of sleep debt and an increased risk of fatigue-related error. In a prior intensive observational study, we found that physicians-in-training working recurrent 24-hour shifts made 36% more serious medical errors and five times as many serious diagnostic errors as those whose scheduled work is limited to 16 consecutive hours. We propose to conduct an intensive study of attending surgeons and obstetricians that will measure intra-operative processes of care and errors. In doing so, we will draw upon both our own experience conducting intensive studies of sleep and medical error, as well as emerging tools to quantify surgical safety and outcomes. Our specific aims will be: 1. To test the hypothesis that the incidence of technical performance problems - as prospectively assessed by direct intra-operative observation, as well as healthcare provider reports - will be significantly higher during surgical cases performed the day following an overnight case (i.e., a case occurring between 12am and 7am) compared with matched cases on days without any preceding night work. 2. To test the hypothesis that the incidence of attentional failures, miscommunications, and teamwork problems - as prospectively assessed by direct intra-operative observation, as well as provider reports - will be significantly higher during surgical cases performed the day following an overnight case compared with matched cases on days without any preceding night work. 3. To test the hypothesis that the incidence of intra-operative serious medical errors (adverse events and near misses) - as assessed by a combination of direct observation, provider reports, and systematic chart surveillance - will be significantly higher during surgical cases performed the day following an overnight case compared with matched cases on days without any preceding night work. The proposed study will build upon our prior studies of sleep deprivation and safety to help develop an understanding of the effects of extended work hours across medical specialties and levels of training.